1. Field of the Invention
The present invention relates to a magnetic oscillator, a magnetic head, and a magnetic recording and reproducing apparatus.
2. Related Art
Since advent of a GMR (giant magneto-resistance) head utilizing a giant magneto-resistance effect (GMR effect), a recording density in magnetic recording has improved at 100% annually. The GMR element is constituted of a stacked film having a sandwich structure of a ferromagnetic layer/a non-magnetic layer/a ferromagnetic layer. The GMR element is a device utilizing a magneto-resistance effect of a so-called spin valve film, which is constituted such that magnetization of one of the ferromagnetic layers is pinned by application of exchange bias to the one and a magnetization direction of the other thereof is changed by applying external magnetic field thereto, so that change in an angle defined between the magnetization directions of the two ferromagnetic layers is detected as a change in resistance value. There have been developed a CIP (current in plane)-GMR element which causes current to flow in a film plane of a spin valve film to detect a resistance change and a CPP (current perpendicular to plane)-GMR element which causes current to flow perpendicularly to a film plane of a spin valve film to detect a resistance change. Both the CIP-GMR element and the CPP-GMR element have a magneto-resistance ratio (MR ratio) of several % or so, and it is considered that both the elements can accommodate a recording density of about 200 Gbit/inch2.
In order to accommodate magnetic recording at a higher density, development of a TMR element utilizing a tunneling magneto-resistance effect (TMR effect) has proceeded. The TMR element comprises a stacked film of a ferromagnetic layer/an insulating layer/a ferromagnetic layer, and it causes a tunnel current to flow in the insulating layer on application of a voltage between the ferromagnetic layers. The TMR element is an element which utilizes such a fact that the magnitude of a tunnel current is changed according to the magnetization directions of the upper and lower ferromagnetic layers to detect change of an angle defined by the magnetization directions as a tunnel resistance value. A TMR element having an MR ratio up to about 50% has been obtained. Since the TMR element has a MR ratio larger than that of the GMR element, its signal voltage becomes larger.
However, there is such a problem that not only a pure signal component but also a noise component due to a shot noise become large, and an S/N ratio (a signal-noise ratio) is not improved. The shot noise is caused by current fluctuation generated due to irregular passing of electrons through a tunnel barrier, and it increases in proportion to square root of a tunnel resistance value. In order to suppress the shot noise and obtain a required signal voltage, therefore, it is necessary to make a tunnel insulating layer thin to lower a tunnel resistance.
Since it is necessary to reduce a device size to a size corresponding to a recording bit or so according to increase in recording density, it is necessary to lower a junction resistance of a tunnel insulating layer, namely, make the insulating layer thin, according to increase in density. A junction resistance of 1Ω·cm2 or less is required in a recording density of 300 Gbit/inch2 and therefore a tunnel insulating layer with a thickness corresponding to a thickness of two atoms must be formed in terms of a film thickness of an Al—O (aluminum oxide film) tunnel insulating layer. Since shortage between the upper and lower electrodes becomes easier to occur according to thinning of the tunnel insulating layer, which leads to reduction of a MR ratio, it becomes exponentially difficult to manufacture an element. Therefore, the limit of the TMR element is estimated to be 300 Gbit/inch2.
The respective elements described above utilize the magneto-resistance effect in a broad sense, but a problem about a magnetic white noise common to these elements has emerged suddenly in recent years. Since the noise is different from an electric noise such as the shot noise described above and is due to thermal fluctuation of magnetization, it is thought that the noise becomes more dominant according to fineness of an element so that the white noise outstrips the electric noise in an element corresponding to 200 Gbpis to 300 Gbpsi. In order to avoid the magnetic white noise and further increase a recording density in magnetic recording, a fine magnetic sensor operating based upon a principle different from the conventional magneto-resistance effect is required, and development of a resonant magneto-resistance effect element has proceeded as such a magnetic sensor (for example, see R. Sato, et. al. J. Magn. Magn. Mat. Vol. 279, p. 36 (2004)).
A characteristic improvement of a conventional resonant magneto-resistance effect element has been promoted by using artificial anti-ferromagnetic material with reduced defects as a magnetic material in a structure where a non-magnetic layer with a thickness of 1 nm or less is sandwiched between ferromagnetic layers whose magnetization directions are perpendicular to a film plane. However, the artificial ferromagnetic material includes many difficult points for practical application due to necessity of a film forming technique with a high level. Therefore, sufficient characteristics can not be obtained currently.
As described above, though development of a novel magnetic sensor utilizing a resonant magneto-resistance effect has proceeded in order to solve the problem about the magnetic white noise adversely affecting the high density magnetic recording, sufficient characteristics for solving the problem have not been achieved yet.